Top U.S. officials warned again Sunday that deadly attacks similar to those in France could happen in the United States, and they raised fresh concerns about potential vulnerabilities in the nation’s counterterrorism system.

Attorney General Eric H. Holder Jr. — speaking in interviews from Paris, where he traveled over the weekend to meet with European counterparts in the wake of attacks that left 17 people dead — discussed the potential for strikes against the United States by individuals or small groups tied to al-Qaeda or other terror organizations.

“It’s something that frankly keeps me up at night, worrying about the lone wolf or a group of people, a very small group of people, who decide to get arms on their own and do what we saw in France,” he said on CBS’s “Face the Nation.” “It’s the kind of thing that our government is focused on doing all that we can, in conjunction with our state and local counterparts, to try to make sure that it does not happen.”

According to several reports, Chérif and Said Kouachi, the two brothers whose attack on the offices of a French satirical newspaper last week left 12 people dead, had ties to al-Qaeda in the Arabian Peninsula and at least one of them had traveled to Yemen. French officials had them under surveillance, with U.S. officials providing information about Yemeni ties, but French security officials had stopped monitoring the brothers months before the attack.

In a series of appearances on the Sunday political talk shows, Holder did not comment on whether the lapse represented an intelligence breakdown by French security officials or a law enforcement resource issue.

“There’ll be time for an after-action analysis of exactly what might have been done better,” he said on NBC’s “Meet the Press.” “You always look back to determine how you might do things better. I will say that the French have been among our best allies, our greatest friends in this fight against global terrorism.”

Holder’s comments came as at least 1.3 million people, including world leaders and dignitaries, flooded the streets of Paris for what French officials described as the largest demonstration in the country’s history.

Several senior lawmakers also warned Sunday of the specter of similar attacks, saying that they would be pressing the Obama administration to keep closer tabs on U.S. citizens and others who travel overseas to train with terrorist groups and then attempt to return home.

Small-scale attacks are “very difficult to detect, deter and disrupt, which is really our goal,” Rep. Michael McCaul (R-Tex.), who leads the House Homeland Security Committee, said on CBS. “I think we’re going to see more and more of these taking place, whether it be foreign fighters going to the warfare and return[ing] or whether it be someone who’s getting on the Internet . . . and then radicalizing over the Internet.”

McCaul and other lawmakers said that the U.S. “no-fly” list remains an important antiterrorism tool but worried about the vulnerability of the visa waiver program, which allows people from designated countries to travel into the United States for up to 90 days without first obtaining a visa.

“The visa waiver program is the Achilles’ heel of America,” Sen. Dianne Feinstein (D-Calif.), a senior member of the Senate Intelligence and Judiciary panels, said during an appearance on CNN’s “State of the Union.”

Although the Kouachi brothers had been placed on the U.S. no-fly list, Feinstein said that other suspects could undergo terror training and slip into the United States undetected through a visa waiver country.

“It’s difficult to ferret someone out,” she added. “There are stolen travel documents in large numbers that they can pick up — pick up a false passport, et cetera. So, we have a big problem there. I think we need to take a look at the visa waiver program again and see what we can do to prevent this kind of thing from happening, because I believe it will happen.”

On Sunday, the White House announced that President Obama on Feb. 18 will host a summit on countering extremism, to highlight how local communities are working to curb violence before it takes place.

The White House did not release a list of expected attendees, but said the meeting will include talks by officials from Boston, Los Angeles and Minnesota’s Twin Cities, who will discuss how they united law enforcement with social-service providers, including mental health professionals and religious leaders, to root out potential violence.

Leaders from the worlds of business and technology are also expected to attend, according to a senior administration official who was not authorized to speak publicly about planning for the event. They are expected to discuss how social media is being used by extremists and possible ways to use such platforms to push back against such groups.

Holder announced plans for the summit during his meetings in Paris and invited world leaders, a move welcomed by French officials. Speaking on ABC’s “This Week,” Gérard Araud, the French ambassador to the United States, called the summit “a very good idea, because, you know, in a sense, France was not attacked as France. France was attacked as a Western democracy.”

Holder said on CBS that federal, state and local officials continue tracking potential terrorism suspects “in an appropriate way, using legitimate tools.”

“We’re not stereotyping anybody, but we are focused on those people who we have some reason to believe might engage in these kinds of these activities,” he said.

Nia-Malika Henderson and Juliet Eilperin contributed to this report.